An optical position-measuring device is described in U.S. Pat. No. 6,831,267. Reference is made in this regard to FIGS. 11 and 50, in particular. The position-measuring device is used for the high-resolution sensing of the relative position of a scanning unit and a measuring standard having a measuring graduation, the scanning unit and the measuring standard being movable relative to each other along at least one measuring direction. The scanning unit includes a light source, a first annular scanning graduation, a reflector element configured directly behind the same, a beamsplitter element, as well as a detection unit. A beam of rays emitted by the light source impinges on the measuring graduation where it is split into at least two partial beams of rays. The partial beams of rays reflected back to the scanning unit are re-reflected by the first scanning graduation and the reflector element in the direction of the measuring graduation, the partial beams of rays propagating, in turn, through the first scanning graduation on the path to the measuring graduation. The first scanning graduation is configured as a circular grating. A circular grating of this kind is composed of radially equidistantly configured, concentric grating bars. Following re-reflection at the measuring graduation, the partial beams of rays propagating in the direction of the scanning unit undergo superposition and are deflected by the beamsplitter element in the direction of the detection unit where a plurality of positionally dependent scanning signals can be recorded. The two partial beams of rays are mutually orthogonally polarized by supplementary optical polarization components in the beam path between the measuring graduation, the measuring standard and the first scanning grating, so that, in response to the relative movement of the measuring standard and the scanning unit, high-resolution, phase-shifted scanning signals can be generated in the detection unit using known optical polarization methods.
Due to the considerable cylindrical symmetry, the optical position-measuring device illustrated in the two mentioned figures of U.S. Pat. No. 6,831,267 supposedly has very high tolerances to tilting of the scanning unit relative to the measuring standard. Particularly in this connection, there is supposedly a particular insensitivity to what is generally referred to as Moiré tilt-angle variations. This is understood to be the tilting of the scanning unit and the measuring standard about an axis of rotation that is oriented normally to the measuring graduation plane.
However, in this type of optical position-measuring device there are various weak points in the discussed scanning optics. For example, the measuring graduation and the first scanning graduation influence the wavefront of the diffracted component beams very differently. In particular, the circular grating of the first scanning graduation distorts the wavefronts considerably since the grating bars are circularly arcuate. A first scanning graduation arranged in this manner is not suited for scanning a linear measuring graduation. Significant wavefront distortions arise in the split partial beams of rays that lead to an extremely low modulation depth of the scanning signals. Since there is a marked increase in the wavefront distortions transversely to the beam direction, a very small cross section needs to be selected for the incident beam coming from the light source. This makes the optical position-measuring device highly sensitive to contamination and defects.
Of even greater concern, however, is that the considerable wavefront distortions in the optical position-measuring device lead to extremely narrow installation, operating and manufacturing tolerances. In the context of such installation, operating and manufacturing tolerances, small lateral shifts in the two partial beams of rays arise. These lead to considerable local phase shifts in response to significant wavefront distortions, and thus to an insufficient interference of the superimposed partial beams of rays. This, in turn, results in a significant decline in the scanning signal intensity. Only in few application cases are the exceedingly narrow installation and manufacturing tolerances ascertained during simulations acceptable. These are the cases which, on the one hand, require a high tolerance to Moiré tilt-angle variations. On the other hand, however, all other tolerances must be significantly narrower than those associated with commercial optical position-measuring devices.